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Pre-Design Investigation Workplan Former Manufactured Gas Plant—Lodi, California borehole the void space will be filled by expansion of the surrounding hydrated bentonite backfill placed in the borehole during probe installation. 4.8 SOIL GAS SAMPLE COLLECTION PROCEDURES Varying field conditions such as rainfall, fine-grained soils, or drilling refusal may affect the ability to collect soil gas samples. If no-flow or low-flow conditions, defined by a flow rate less than 100 milliliters per minute (mL/min) under an applied vacuum of 100 inches of water column (" W.C.), are caused by wet soils resulting from a rain event,irrigation water, or shallow groundwater, the soil gas sampling will cease. In addition, soil gas sampling will not be conducted during or within a minimum of 5-days following a significant rain event (rainfall of '/z -inch or more). If low-flow conditions are determined to be from the presence of fine-grained soils, alternate low flow sampling methods outlined in Appendix D of the guidance (DTSC, 2010) will be conducted, or a new probe may be installed at a greater depth or at a new alternate location following evaluation of the available lithologic logs. 4.8.1 Equilibration Time Following installation of the soil gas probe and prior to soil gas sample collection activities (purging, leak test, soil gas sampling), subsurface conditions will be allowed to equilibrate for a minimum period of 30 minutes, in accordance with the sampling protocols in the ASGI guidance document for probes installed with a direct-push drill rig (DTSC, 2010). Additionally, all soil gas probes installed within hand auger boreholes (i.e., 5-foot probe depth) will be allowed to equilibrate for a minimum period of 48 hours,in accordance with the DTSC ASGI guidance (DTSC,2010). 4.8.2 Leak Test Leak testing will be conducted at each soil gas sampling probe to evaluate whether a good seal has been established in the sampling train, ground surface, and probe interface. Leak testing consisting of a "vacuum shut-in test" to test the aboveground sampling equipment's ability to hold vacuum in the tubing,valves, and connections located between the soil gas probe and Summa7 canister and a chemical tracer leak test using a leak-check compound (e.g., helium) will be conducted prior to soil gas sample collection at each soil gas probe to test the integrity of the sampling system, probe, and borehole seal. The "vacuum shut-in test" will be conducted at every soil gas probe prior to soil gas purging to verify there are no leaks in the aboveground sample train equipment and connection points, which includes the valves, tubing, and fittings between the soil gas probe and the Summa canister. This test consists of applying a vacuum of at least 100"W.C. to the tubing and valve system between the soil gas probe and Summa'"' canister, closing the valves to seal the vacuum in the line, and verifying that the vacuum (100" W.C.) is maintained for at least 60 seconds in the line. If the vacuum is not maintained for 60 seconds, the tubing and valves will be changed and/or adjusted, and the test conducted again until the vacuum can be maintained for a minimum period of 60 seconds indicating a leak is not occurring in the aboveground sample train equipment and connection points. TERRA PACIFIC GROUP Page 15 April 4, 2012